It could be a blister, an infection, or even an ulcer. I suggest you have your doctor take a look to rule out something serious. Not to alarm you, but it is sometimes possible that it could be cancer.
Are you a smoker?
Okay, good! Then this is probably a scab or a blood blister resulting from your sore throat. You probably have strep. I would see the doctor for a throat culture tomorrow.
Oh my, probably not. It is probably less effective than it would be if it were fresher.
If you can tell me what city you are in, I can try to find a minute-clinic for you. These are in some pharmacies. You can see a Nurse Practioner with a Masters in Family Medicine, and get diagnosed and medicine in one spot. The price is reasonable as well.
Oh shoot, there isn't one of these clinics in CA or in TX. I'm sorry! It's only in about 7 states so far.
I'll bet your doctor will give you a call back. In the mean time, make sure you are drinking lots of water, and take some ibuprofen for any pain or fever.
I hope you are feeling better real soon!